1. Field of the Invention
The present invention relates to an ATM communication device using interface signals of an HIPPI (High Performance Parallel Interface) and, more particularly, to an ATM communication device capable of efficiently transferring data packets at a high speed.
2. Description of the Related Art
HIPPI is an electric signal interface for conducting high-speed data transfer to transmit data at 800 Mbps (megabits per second) or 1.6 Gbps (giga bits per second) of information. Interface signals of an HIPPI are mainly used in communication conducted by such systems as super computers and work stations dealing with graphic data which require high-speed data transfer. For efficient execution of such high-speed data transfer, an ATM (Asynchronous Transfer Mode) line needs at least a line of STM-4 (Synchronous Transfer Module-4 (622 Mbps) recommended by ITU-T (International Telecommunication Standardization Sector)) with a capacity of 622 Mbps or a super-high-speed line with a larger capacity. 622 Mbps capacity STM-4 line is, however, too expensive as a subscriber's loop of an ATM network to be widely used.
FIG. 6 is a timing chart showing the state of data transfer by a conventional ATM communication device using an HIPPI interface. In data transfer under the credit control conducted based on the recommendation of ANSI (American National Standards Institute), upon generation of a transmission request, a device on a transmission side transmits a connection request packet 11 to a device on a data reception side through an ATM line. Here, credit control is referred to as one of techniques for controlling transmission by a device on the transmission side so as to prevent an overflow of data from occurring at the device on the reception side. More specifically, credit control is realized by timely notifying a device on a transmission side by a device on a reception side, of a credit value indicative of the number of receivable packets at that time on the reception side device. Packet in this case represents a unit data length as a unit of transmission and reception, which does not always coincide with a packet in protocol (transmission procedure).
When the data reception side device which has received the connection request packet 11 is allowed to receive data, it returns a packet 12 including initial credits to the transmission side device. A credit value notified to a transmission side device by a reception side device at the beginning of a series of communication is referred to as an initial credit value. Ordinarily used as an initial credit value is a value indicative of the number of buffers corresponding to the number of maximum continuous bursts receivable by the reception side device. Burst has a fixed length and in general a plurality of bursts constitutes one packet with a variable length. In a case of an HIPPI, while an HIPPI packet in protocol is made up of a plurality of HIPPI bursts and a transmission destination is set on an HIPPI packet basis, credit control is applied on an HIPPI burst basis. In other words, an object of credit control in the HIPPI is an HIPPI burst.
Upon receiving the packet 12 including the initial credits, the data transmission side device is allowed to continuously transmit as many HIPPI bursts as the number of the initial credits. In FIG. 6, one arrow 13 is illustrated to correspond to one HIPPI burst.
When the data transmission side device starts transmission of an HIPPI packet, the data reception side device, upon receiving as many HIPPI bursts as the number equivalent to a new predetermined credit value, returns a packet 14 including the new credits to the transmission side device. As a result, the reception side device notifies the transmission side device that as many HIPPI bursts as the number equivalent to the new credit value (in a case of FIG. 6, the new credit value is set to be "8") are receivable. Notification by the transmission of a packet including new credits will be repeated until transmission of all the HIPPI bursts from the transmission side device is completed.
After the above-described control is repeated until all the HIPPI bursts are sent from the transmission side device to the reception side device, a cut-off instruction packet 15 for instructing to cut off data transmission is transmitted from the transmission side device to the reception side device.
In a conventional ATM communication device, a communication line on the ATM side is composed of high-speed ATM lines, one for transmission and the other for reception, as described in the foregoing. In other words, the conventional device is structured such that both physically and logically, independent transmission and reception HIPPI interfaces correspond one-to-one to a transmission line and a reception line on the ATM side. Conventional techniques of this kind are disclosed, for example, in Japanese Patent Laying-Open (Kokai) No. Heisei 4-220835, entitled "Flow Control System in Wide-band ISDN" and Japanese Patent Laying-Open (Kokai) No. Heisei 5-22334, entitled "ATM Cellulation Mode of HIPPI".
Recited in Japanese Patent Laying-Open No. 4-220835 is a flow control system structured, so as to improve efficiency of data transfer of HIPPI without an effect of a delay in an ATM network, such that terminal adapters for an HIPPI on the transmission and reception sides are provided with buffers for accumulating a predetermined number of cells and such that before receiving a ready signal from the reception side, the transmission side terminal adapter sends ready information corresponding to the number of bursts which can be stored in the buffer to the transmission side terminal to hold transmittable bursts irrespective of a ready signal from the reception side.
Recited in Japanese Patent Laying-Open No. 5-22334 is an ATM cellulation mode of an HIPPI in which, at the time of transmitting, in an ATM cell, HIPPI data and parity of which 4-byte data and 0.5-byte parity constitute one word, cellulation is conducted on a basis of two words, with an odd-numbered word and an even-numbered word as a pair, in order to efficiently conduct ATM transmission in a format of a payload of a cell at the time of ATM-cellulation of HIPPI data and parity while saving unused worthless bits.
With the above-described corresponding relationship between an HIPPI interface and an ATM, used as a communication line is a common 155 Mbps capacity STM-1 line which sacrifices transfer efficiency or an expensive and rare 622 Mbps capacity STM-4 line. However, even the 622 Mbps capacity STM-4 line is too short in capacity for a communication rate of 800 Mbps at the HIPPI interface to conduct efficient ATM communication using interface signals of an HIPPI at a high speed. Although there is another line with a capacity of 2.4 Gbps as an ATM line, it is mainly used as a trunk line and not for direct connection between subscribers and an ATM network.
Conventional ATM communication devices have a shortcoming, as described in the foregoing, that use of a common 155 Mbps capacity STM-1 line causes the device to have a reduced communication rate and accordingly prevents the device from making the most of a high communication rate of an HIPPI.
On the other hand, with a 622 Mbps capacity STM-4 line, while preferable communication rate is obtained as compared with a STM-1, it is not high enough as compared with 800 Mbps of the HIPPI. Moreover, because of its high cost, use of the 622 Mbps capacity STM-4 line is not economical.